The universal serial bus (USB) has a variety of operating modes that allow a number of computer peripherals to be connected to a generic port. One of the modes of a USB device is a low speed mode. While operating in the low speed mode, the outputs of a USB device are generally very slow transitioning signals. Another design criteria involved with USB devices is that there may be a large variation in the load placed on the device. As a result, the USB device must be designed to provide the proper speed at the output across wide load variations.
Referring to FIG. 1, a conventional USB driver circuit 10 is shown which implements operational amplifiers to control the transitioning of an output signal. The circuit 10 generally requires the use of an operational amplifier 12 and an operational amplifier 14 to control the signal presented at the output pad 16. The circuit 10 also requires a variety of transistors, a bias circuit 18, a predriver circuit 20 and a clamp circuit 22. More information regarding the circuit of FIG. 1 may be found in the paper "Design Guide For A Low Speed Buffer For The Universal Serial Bus", published in December of 1996, the appropriate sections which are hereby incorporated by reference.
Each universal serial bus device has two outputs, a plus output and a minus output. The circuit 10 would have to be implemented, at a minimum, at each of the plus and minus outputs. In an application that provides a number of universal serial bus outputs on a single device, the number of instances that the circuit 10 would have to be duplicated increases accordingly. As a result, it is desirable for the number of components in the circuit 10 be kept to a minimum in order to reduce the overall area required to implement the plurality of required buffers at the various outputs.